At present, magneto-optical disks, write-once optical disks, and compact disks (CDs) are already commercialized as optical disks for recording and reproducing information via irradiation of laser beams. There is, however, increasing interest in digital video disks (DVDs) as the next generation recording medium.
An optical pickup is a device for recording and reproducing information to and from optical disks. Increasing importance is now being placed on development of technology for miniaturization of optical pickups in response to the increasing trend towards integration.
One example of an optical pickup is Japanese Patent Application H7-136462 which discloses a small optical pickup for magneto-optical disks shown in FIGS. 11A and 11B. FIG. 11A is a simplified sectional view of an optical system of the above prior art. FIG. 11B is a top view of a light receiving element, light emitting element, and analyzer. In FIGS. 11A and 11B, a substrate 82 is provided inside an optical module 81. A laser diode 83 as the light emitting element and photo detectors 84, 85, and 86 as the light receiving element are disposed on the substrate 82. The laser diode 83 has a structure which allows, for example, a concave portion having a 45.degree. angled plane (not illustrated) to be disposed on one part of the substrate 82, and a light emitting chip (not illustrated) to be disposed inside that concave portion for reflecting light radiated from the light emitting chip on the 45.degree. angled plane, thus routing the beam upward. Each of the photo detectors 84 and 85 consists of six components: 84a to 84f and 85a to 85f, respectively. The photo analyzer 86 consists of two components: 86a and 86b which are inclined approximately 45.degree. with respect to the array direction of the photo detectors 84 and 85.
The transparent substrate 87 is made of glass or resin, and has a hologram diffraction grating 88 on the side facing the laser diode 83. The hologram diffraction grating 88 has a lens effect, giving different focal lengths to .+-. primary diffraction light which have been diffracted between approximately 5.degree. and 20.degree.. The transparent substrate 87 is provided over the optical module 81 to seal the inside of the optical module 81. A polarizing prism 89 has a trapezoidal cross-section, formed by bonding a triangular prism having a right triangle cross-section and an approximate parallel prism having an approximate 45.degree. cross section. A polarization beam splitter film consisting of multiple layers of a range of thin dielectric films as shown in Table 1 is applied to a bonded portion 89a of the crystal polarizer 89 in such a way, for example, that the transmittance of p-polarized light is approximately 70%, the reflectance of p-polarized light is approximately 30%, and the reflectance of s-polarized light is approximately 100% when p-polarized light is emitted from the laser diode 83.
TABLE 1 ______________________________________ Film thickness ______________________________________ Substrate n = 1.635 -- 1st layer TiO.sub.2 119 nm 2nd layer SiO.sub.2 183 nm 3rd layer TiO.sub.2 119 nm 4th layer SiO.sub.2 183 nm 5th layer TiO.sub.2 119 nm 6th layer SiO.sub.2 183 nm 7th layer TiO.sub.2 119 nm 8th layer SiO.sub.2 183 nm 9th layer TiO.sub.2 119 nm Substrate n = 1.635 -- ______________________________________
The polarizing prism 89 is integrated onto the transparent substrate 87, and an angled plane 89b inclines toward the inside of the optical module 81. A reflection film consisting of multiple layers of a range of thin dielectric film as shown in Table 2 is applied to the surface of the angled plane 89b.
TABLE 2 ______________________________________ Film thickness ______________________________________ Substrate n = 1.635 -- 1st layer TiO.sub.2 119 nm 2nd layer SiO.sub.2 183 nm 3rd layer TiO.sub.2 119 nm 4th layer SiO.sub.2 183 nm 5th layer TiO.sub.2 119 nm 6th layer SiO.sub.2 183 nm 7th layer TiO.sub.2 119 nm 8th layer SiO.sub.2 183 nm 9th layer TiO.sub.2 119 nm l0th layer SiO.sub.2 183 nm 11th layer TiO.sub.2 119 nm 12th layer SiO.sub.2 183 nm 13th layer TiO.sub.2 119 nm 14th layer SiO.sub.2 183 nm 15th layer TiO.sub.2 119 nm 16th layer SiO.sub.2 183 nm 17th layer TiO.sub.2 119 nm 18th layer SiO.sub.2 183 nm 19th layer TiO.sub.2 119 nm 20th layer SiO.sub.2 183 nm 21st layer TiO.sub.2 119 nm 22nd layer SiO.sub.2 183 nm 23rd layer TiO.sub.2 119 nm 24th layer SiO.sub.2 183 nm 25th layer TiO.sub.2 119 nm 26th layer SiO.sub.2 183 nm 27th layer TiO.sub.2 119 nm 28th layer SiO.sub.2 183 nm 29th layer TiO.sub.2 119 nm 30th layer SiO.sub.2 183 nm 31st layer TiO.sub.2 119 nm 32nd layer SiO.sub.2 183 nm 33rd layer TiO.sub.2 119 nm Substrate n = 1.635 -- ______________________________________
A prism analyzer 90 has a trapezoidal cross-section, formed by bonding a prism having a triangular cross-section and a prism having a parallelogram cross-section. The prism analyzer 90 has a polarization splitting plane 90a at the junction plane of the above two prisms which is set to transmit approximately 100% of p-polarized light and reflect approximately 100% of s-polarized light. The prism analyzer 90 is disposed on the substrate 82 over the photo detector 86. The polarization splitting plane 90a is located over the component 86a of the photo detector 86, and an angled plane 90b is located over the component 86b of the photo detector 86. An object lens 91 is disposed over the polarizing prism 89 to focus the light.
In the magneto-optical pickup as configured above, p-polarized light emitted from the laser diode 83 is transmitted through the transparent substrate 87 on which the hologram diffraction grating 88 is formed, and enters the polarization splitting plane 89a of the polarizing prism 89. Since the polarization splitting plane 89a is set to transmit approximately 70% of p-polarized light, reflect approximately 30% of p-polarized light, and reflect approximately 100% of s-polarized light; approximately 70% of entering light is transmitted and focused by the object lens 91 on the magneto-optical recording medium 92. The polarization plane of light is rotated about 0.5.degree. on the magneto-optical recording medium 92, depending on the type of recorded magnetic signals, for reflecting the light incorporating a small quantity of s-polarized light component as the magneto-optical signal component, following which the reflected light is transmitted through the object lens 91 again, and back to the polarization splitting plane 89a of the polarizing prism 89.
The polarization splitting plane 89a is set to transmit approximately 70% of p-polarized light, reflect approximately 30% of p-polarized light, and reflect approximately 100% of s-polarized light. Thus, approximately 70% of the p-polarized component is transmitted, and approximately 30% of the p-polarized component and 100% of the s-polarized component, which is the magneto-optical signal component, are reflected. Here, the components reflected on the polarization splitting plane 89a is reflected onto the angled plane 89b, passes through the transparent substrate 87 into the optical module 81, and enters the polarization splitting plane 90a of the prism analyzer 90. Since the polarization splitting plane 90a is set to transmit approximately 100% of p-polarized light and reflect approximately 100% of s-polarized light, the p-polarized light component passes through the polarization splitting plane 90a and enters the part 86a of the photo detector 86; the s-polarized component is reflected on the polarization splitting plane 90a, reflected on the angled plane 90b, and then enters the component 86b of the photo detector 86.
In the magneto-optical pickup as configured above, the polarizing prism 89 with different refractivity and transmittance for p-polarized light and s-polarized light is integrated into the optical module 81. The laser diode 83 and the photo detectors 84 to 86 are provided inside the optical module 81, and the prism analyzer 90 is integrated into the substrate 82. This configuration realizes a smaller, lower cost, and integrated optical pickup for magneto-optical disks.
Japanese Patent Application H7-188898 discloses a small optical pickup for DVDs as shown in FIGS. 12A and 12B. FIG. 12A shows a simplified sectional view of an optical system of the prior art, and FIG. 12B shows a magnified top view of a light receiving element.
In FIGS. 12A and 12B, an optical module 103 has a substrate 104 inside. A laser diode 105 and photo detectors 106, 107, and 108 are disposed on the substrate 104. The laser diode 105 is provided, for example, with a concave portion (not illustrated) having a 45.degree. angled plane on a part of the substrate 104. A light emitting chip (not illustrated) is mounted inside, and light emitted from the light emitting chip is reflected on the 45.degree. angled plane and radiated upwards. The laser diode emits linearly p-polarized light. The photo detectors 106 and 108 consist of four areas: 106a, 106b, 106c, and 106d; and 108a, 108b, 108c, and 108d respectively. The direction of the line that splits the areas 106b and 106d, and areas 108b and 108d is approximately parallel to the direction of the information tracks of a information recording medium 124. The transparent substrate 109 is made of glass or resin, and has a hologram diffraction grating 120 on the side facing the photo detectors 106, 107, and 108. A hologram diffraction grating 120 has a lens effect, giving .+-. primary diffraction light diffracted between approximately 5.degree. and 20.degree. to be focused close to and far from the transparent substrate 109 respectively, centering on the plane of the photo detectors 106, 107, and 108. The transparent substrate 109 is disposed to seal the inside of the optical module 103.
A polarization beam splitter 121 is formed by bonding a prism having an approximate right triangle cross-section and an approximate parallel prism having an approximate 45.degree. cross-section. An optical film 121a which transmits p-polarized light and reflects s-polarized light is applied to the junction plane of the two prisms. The optical film 121a consists of multiple layers of multiple dielectric films as shown in Table 3.
TABLE 3 ______________________________________ Film thickness ______________________________________ Substrate n = 1.51 -- 1st layer TiO.sub.2 97 nm 2nd layer SiO.sub.2 152 nm 3rd layer TiO.sub.2 97 nm 4th layer SiO.sub.2 152 nm 5th layer TiO.sub.2 97 nm 6th layer SiO.sub.2 152 nm 7th layer TiO.sub.2 97 nm 8th layer SiO.sub.2 152 nm 9th layer TiO.sub.2 97 nm Substrate n = 1.51 -- ______________________________________
The polarization beam splitter 121 is disposed on and integrated into the transparent substrate 109. An angled plane 121b inclines towards the inside of the optical module 103. A quarter-wave plate 122 is disposed on the surface of the polarization beam splitter 121 in an integrated fashion, where it converts linearly polarized light to circularly polarized light.
In the optical pickup as configured above, p-polarized light emitted from the laser diode 105 passes through the transparent substrate 109, enters the polarization beam splitter 121, passes through the optical film 121a, and enters the quarter-wave plate 122. The p-polarized light is converted to the circularly polarized light in the quarter-wave plate 122, and the circularly polarized light is focused on the information recording medium 124 by an object lens 123. The circularly polarized light reflected upon receiving the information signal on the information recording medium 124 passes through the object lens 123 again, and enters the quarter-wave plate 122. Here, the circularly polarized light is again converted to linearly polarized light which orthogonally crosses the p-polarized light emitted from the laser diode 105, i.e. s-polarized light, in the quarter-wave plate 122. The s-polarized light then enters the polarization beam splitter 121, is reflected on the optical film 121a and on the angled plane 121b, and enters the hologram diffraction grating 120 of the transparent substrate 109.
Here, the light is diffracted at diffraction angles of approximately 5.degree. to 20.degree.. The + primary diffraction light, for example, enters the photo detector 106, zero diffraction light enters the photo detector 107, and the - primary diffraction light enters the photo detector 108.
In the configuration of the optical pickup for DVDs as explained above, the optical module 103 and polarization beam splitter 121 are integrated, and the polarization beam splitter 121 and quarter-wave plate 122 are also integrated, offering drastically smaller optical pickups. Furthermore, the number of components has been reduced by integrating the laser diode 105 and photo detectors 106, 107, and 108 into the optical module 103; and manufacturing costs have been reduced due to lack of need for extreme precision in the positioning of the photo detectors 106, 107, and 108.
Integration of components of optical pickups is achieved by employing a semiconductor laser which essentially generates diffused light, spreading to a certain range, as a light source. Therefore, the optical characteristics of components of optical pickups are strongly affected by the incident angle.
In the optical pickup for magneto-optical disks shown in FIGS. 11A and 11B, a phase difference may occur between the p-polarization light and s-polarization light responsive to broad incident angles because the polarization beam splitter film and reflection film disposed respectively on the face 89a of a glass material having an approximate parallelogram cross-section and the plane 89b which is approximately parallel to the plane 89a only consist of dielectric films.
FIG. 13 shows the phase difference between the p-polarized light and s-polarized light (hereafter referred to as the "p-s phase difference") of light reflected on the polarization beam splitter film, the p-s phase difference of light reflected on the reflection film, and the total of the p-s phase difference of the polarization beam splitter film and of the reflection film. This explains the dependence of the p-s phase difference on the incident angle when the light reflected on a disk enters a photo detector. As shown in FIG. 13, the p-s phase difference of the polarization beam splitter film is -50.degree. to +40.degree. when the light incident angle is .+-.10.degree. with respect to the prism, therefore 45.degree..+-.6.degree. with respect to the optical film. The p-s phase difference of the reflection film exceeds -50.degree. to +50.degree. and the p-s phase difference of the light entering the photo detector (shown as the total p-s phase difference in FIG. 13) also exceeds -50.degree. to +50.degree..
To satisfactorily reproduce the information recorded on a magneto-optical disk, the p-s phase difference when the light enters the photo detector may require to be within -20.degree. to +20. With the polarization beam splitter film and reflection film of the prior art, it may be difficult to design an optical pickup which achieves the above preferable range for diffused light.
In the optical pickup for DVDs as shown in FIGS. 12A and 12B, transmittance of the p-polarized light in the polarization beam splitter film greatly depends on the incident angle, as shown in FIG. 14, when the light enters at a wide incident angle (the incident angle to the optical film is 45.degree..+-.7.degree. when the incident angle to the prism is 1.degree..+-.0.degree.). This may cause unsatisfactory reproduction due to reduced light entering the light receiving area.
Furthermore, since the hologram diffraction grating 120 is disposed on the transparent substrate 109 which seals the inside of the optical module, it may be necessary to broaden the diffraction angle .theta. of the diffracted light, i.e. to narrow the pitch of the diffraction grating, when the distance between the hologram diffraction grating 120 and photo detectors 106, 107, and 108 is short. This may cause difficulties in manufacturing such diffraction gratings, resulting in failure to achieve a satisfactory optical pickup.